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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

ALLOYS OF GOLD AND COPPER

289

is carefully decanted off as completely as possible without allowing the
cornet to escape. The residue is again boiled for 10 minutes with 20 c.c.
of nitric acid (D 1-3). With the more concentrated acid the boiling is less
regular and dangerous bumping may occur, and some authorities recommend
the addition of a scrap of wood charcoal or a completely charred pepper-
corn. After this second portion of acid has been decanted off, a further
quantity of 20 c.c. of nitric acid (D 1-3) is added and boiled for 10 minutes
to remove the last traces of silver. When this last acid together with the
piece of charcoal have been removed, the cornet is washed with two quan-
tities of 30-40 c.c. of boiling water, the flask being subsequently filled
completely with cold distilled water. The mouth of the flask is then closed
by means of an inverted crucible of refractory earth or unglazed porcelain,
which is pressed firmly on while the flask is inverted. A little water descends
into the crucible and forms a hydraulic seal, while the brittle and slender
cornet slowly falls to the bottom of the crucible. After some time, when
any small fragments detached from the cornet have been deposited, the
flask is gradually raised to the edges of the crucible, displaced a little laterally
and with a rapid movement brought into an erect position. The water is
decanted from the crucible which is dried on the platform of the furnace
and subsequently heated to redness in the muffle for 2-3 minutes. Under
the influence of the heat the cornet contracts to about one-third of its original
volume and assumes a golden-yellow metallic appearance. When cold, the
two cornets are weighed together, the total weight giving directly the
fineness of the sample ; the weights of the separate cornets should not
differ by more than 0-5 milligram.

In the assay of gold by cupellation, small losses occur (according to
Rose, 0-5-1 on the fineness) owing partly to volatilisation of the gold and
partly to imbibition by the cupel. This slight loss is, however, compen-
sated by the small amount of silver (0-75-1 one-thousandth) which always
remains with the gold in spite of the different treatments with nitric acid.
Thus, according to Rose, if the operation is properly conducted, the error
should not exceed+o-2 per thousand. The losses by volatilisation increase
with the amount of lead used and, consequently, with diminution of the
proportion of gold in the alloy ' on this account, Riche advises the omission
of the third treatment with nitric acid in the case of gold-copper alloys of
lower fineness than 800. In some laboratories the small errors are esti-
mated by making a control assay with pure gold 1 and pure copper in about
the same proportions as in the sample, the mixture being cupelled with
the same quantities of silver and lead, and the parting carried out under
the same conditions. In assaying commercial fine gold Riche advises the
addition of 20 per thousand of copper to prevent brittleness in the button
obtained.

As regards extraneous metals, small quantities of platinum render the

one series and the next are the bottles with the nitric acid and the distilled water and
below them three bottles to receive : (i) the first two lots of acid, rich in silver nitrate,
(2) the third lot of acid, and (3) the wash water.

1 For the preparation of pure gold, see Roberts-Austen : Fourth Mint Report,
London, 1873, 46.

A.C.                                                                                                                  19lyiique, II, p. 561. crown of the muffle.l and ether, dried at